Low-water cut-off system

ABSTRACT

A low-water cut-off system [A] determines if fluid [W] within a boiler [ 16 ] or other enclosure drops below a predetermined level within the enclosure. A signal generator introduces a signal into the enclosure so that the signal is present for sensing at the predetermined level, where a probe [ 14 ] is provided for sensing the signal if present as occurs if the water is at the predetermined level. A control [ 18 ], which includes a microcontroller [ 25 ], is responsive to the probe for providing a control function in response to whether the signal is so sensed by the probe, and so to determine if water has dropped below the predetermined level. The function may be causing alarm signalling or boiler heating cut-off. Self-test and time delay features are also included. Provision is made so that the control compensates for variation in electrical conductivity of the water through which the probe signal is sent.

This application claim benefit of Provisional No. 60/293,444, filed May24, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fluid level detection systems and, moreparticularly, to an advantageous low-water cut-off system that operatesin any of a wide range of conditions and provides precise leveldetection with an extraordinarily wide variety of electricallyconductive fluids.

Although the term low water cut-off system is herein used, it will beunderstood as being applicable to systems using fluids other than waterwhich exhibit electrical conductivity that can be sensed for the presentpurposes. The term low water cut-off system is used in its generalsense, regardless of the fluid to be sensed.

2. Known Art

Low water cut-off systems have long been used in low-pressure and hotwater boilers to cut off a burner for the boiler if there is a low waterlevel (or other low fluid level) condition. Such low water cut-offsystems may be used either for primary or secondary protection. A widevariety of such low water cut-off systems have been known or proposed,based typically either on float level detection or on electricalconductivity of a fluid whose level is to be measured.

Among the challenges in the design of low water cut-off systems are thefollowing requirements:

The system should be preferably adjustable to provide preselected delayin its make or break operation. For example, fluid level in a boiler mayfluctuate during operation, causing a possibility of possibly spurious,inaccurate or premature operation. A preselected delay may accordinglybe preferred before the low water cut-off system closes control contacts(delay on make operation). Further, in the event of operation of the lowwater cut-off system, and/or a preselected delay may be preferred beforethe low water cut-off system opens control contacts (delay on breakoperation).

Moreover, the nature of the fluid (such as water in a boiler system) maybe different from one installation to the next, being dependent, forexample on impurities or additives in the fluid. Electrical conductivityof the fluid to be sensed by the low water cut-off system may be subjectto a wide possible range of purity levels of the water disposed in theboiler tank.

Because low water cut-off systems of the type operating according toelectrical conductivity of water or other liquid require the fluid to beconductive in order to sense its presence, high purity water, whichexhibits very low electrical conductivity, can fool the device into afalse low water condition. Previously, individual or special models hadto be offered to the industry to handle these pure water applications.

Accordingly, it is preferred that a low water cut-off system using aprobe responsive to electrical conductivity be capable of adjustment inprobe sensitivity; however the prior art has not provided a satisfactorylow water cut-off system with suitable probe arrangement in which theprobe is responsive to electrical conductivity. Preferably, the lowwater cut-off system operates under microprocessor control and iscapable of ready, convenient adjustment in probe sensitivity over asuitable range.

In general, there is also a continuing need in the art for a low-watercut-off system that reliably prevents false or premature cut-offoperation in the event of temporary changes or surges in fluid level.There is a further need in the art for a low-water cut-off system thatallows selective easily-set control of both make and break times duringoperation, and that avoids spurious, inaccurate or premature operationof the system.

Within the art, there also is a general failure to teach a low watercut-off system which could allow for rapid and convenient adjustment ofthe conductivity level of the fluid with which the probe is used inorder to account for the purity level of the water or other fluid in theboiler tank.

Finally, there is a need in the art for a low-water cut-off system thatprovides combinations of these features for use in a wide variety oflow-pressure steam and hot water boilers, as well as the capability forbeing used in both manual and automatic reset modes.

Among the many patented low water cut-off system systems, none can befound that appears more pertinent or relevant than U.S. Pat. No.5,111,691 to John et al. which discloses a low water cut off conductanceprobe and control system having a time delay feature. The probe combinestemperature and electrical conductivity sensing operations. However, thetime delay feature utilized in said John et al. reference is fixed suchthat the system cannot be adjusted by the user without changing out theparticular hardware arrangement described in the reference. The timedelay feature of that patent has an R-C (resistive-capacitance) chargingcircuit in combination with an analog timer device that turns on and offan output relay to the boiler when charged or discharged by the chargingcircuit. More specifically, when the probe permits current flow throughwater filling the tank of the boiler the charging circuit begins tocause charging of a capacitor of a timing circuit. After a fixed periodof time, the capacitor voltage reaches a level to enable an analog timerdevice which in turn activates the boiler through an output relay.Conversely, when the water level drops below a predetermined level, theprobe no longer senses the electrical charge and the charging circuitdischarges the capacitor over a fixed period of time. When the chargingcircuit is fully de-energized the analog timer device turns off theboiler.

In contrast to the John et al. reference, the present invention providesa low water cut-off system having an adjustable time delay featureimplemented by a microprocessor, rather than being part of a fixedhardware circuit arrangement as found in the John et al. reference.

In further contrast with said John et al. reference, the low watercut-off system of the present invention allows a user to simply set thepreferred time delay for the system at the microprocessor in order toadjust the time delay. The John et al. reference does not teach norsuggest such an adjustable time delay system much less one which ismicroprocessor-implemented and readily adjustable in time delay as wellas other operational parameters.

Moreover, the John et al. reference does not teach nor suggest theprovision of the new low water cut-off system according to the presentinvention for setting or adjusting the probe conductivity to the puritylevel of the water or other fluid in a boiler.

OBJECTS AND SUMMARY OF THE INVENTION

Among the several objects, features and advantages of the invention maybe noted the provision of an improved low water cut-off system which:

-   -   is responsive to electrical conductivity of water or other fluid        used in a boiler or other device or system where fluid level        must be sensed;    -   provides utility and precision as well as highly reliable        operation both for low-pressure and hot water boilers to cut off        a burner for the boiler if there is sensed a low water level (or        other low fluid level) condition;    -   can be used either for primary or secondary protection;    -   has an adjustment feature that compensates for less conductive,        pure water applications, and so provides a distinct advantage        over prior low water cut-off systems in that the low water        cut-off system of the present invention is capable of serving        the industry as a single control which can cover a larger        variety of water conditions;    -   can be used over a wide range of electrical conductivity values        of fluid to be sensed by allowing selective adjustment for use        in sensing a fluid with a known purity level, as when there are        impurities or additives in the fluid;    -   uses a highly precise probe and electronic circuitry responsive        to electrical conductivity, yet capable of adjustment precisely        and easily in probe sensitivity;    -   reliably prevents false or premature cut-off operation in the        event of temporary changes or surges in fluid level; is        microprocessor-based for solid state reliable and facile        adjustment;    -   provides operation and different possible input supply voltages,        such as preferably 120 VAC or 24 VAC;    -   operates with very low electrical power consumption;    -   permits different possible probe lengths;    -   can be used for either indoor or outdoor use;    -   can be used in hazardous locations as well as in remote        locations;    -   allows selective easily-set control of both delay on make (DOM)        and delay on break (DOB) times in operation;    -   avoids spurious, inaccurate or premature operation;    -   provides for fail-safe operation to prevent undesired lock out        upon power loss;    -   includes features for assuring of reliable fault-free operation;        and    -   is capable of being specified for use in either manual and        automatic reset modes.

In a general sense, the low water cut-off system of the presentinvention is an apparatus for determining the presence of a fluid at apredetermined level within a fluid-containing enclosure in which thefluid can rise to the predetermined level. The apparatus operatesaccording to a method comprising the introduction, of a signal into thefluid-containing enclosure such that the signal is present for sensingwithin the fluid at the predetermined level, sensing for the presence ofthe signal within the fluid at the predetermined level, and providing acontrol function in response to whether the signal is so sensed, inorder to indicate whether the fluid is present at the predeterminedlevel.

In brief, it will be seen that the present invention relates to a lowwater cut-off system for determining whether the water has dropped belowa predetermined level within the fluid-containing enclosure, wherein thesystem comprises:

a signal generator operative to introduce a signal into thefluid-containing enclosure such that the signal is present for sensingwithin the fluid at the predetermined level, a probe at thepredetermined level capable of sensing the presence of the signal withinthe fluid if the fluid is at the predetermined level, and a controlresponsive to the probe sensing for providing a control function inresponse to whether the signal is so sensed by the probe in order toindicate whether the water has dropped below the predetermined level.

More specifically summarized, the low water cut-off system of thepresent invention is a microprocessor-based control system for detectingthe presence of an electrically conductive fluid in a vessel or tank,such as a boiler. It includes a probe for signaling a low watercondition in the boiler or other vessel or tank. The system is providedwith an adjustable time delay feature for preventing rapid shuttling ofthe device between on and off conditions whenever the water level isfluctuating about the probe. It also includes a water purity featurethat permits the user to change the conductance level of the probe inorder to increase detection sensitivity when accounting for higherlevels of water purity in the boiler in accordance with the electricalconductivity of the fluid. The system includes a housing containingelectronic circuitry and the probe, which is operatively connected tothe electronic circuitry and extends outwardly from a brass bushingcarried by the housing. The electronic circuitry includes amicroprocessor which permits the user to readily and easily adjust thetime delay without having to change the hardware arrangement of thesystem. In operation, an electric current is applied to the device whichflows through the housing and into the conductive fluid, such as water,that surrounds the housing. Whenever the fluid level is above the probe,the electric current flows through the conductive fluid and the probe.However, if fluid level falls below the probe, the probe no longerdetects electric current generated by the device through the conductivefluid. An alarm signal is then transmitted to a remote site signalingthe fact that a low fluid (low water) condition exists. Additionally,the system also shuts down the boiler whenever a low water conditioncontinues to exist for a predetermined amount of time. The systempermits preselected delay to set for either its or break operation.

Additional objects, advantages and novel features of the invention willbe set forth in the description which follows or will become apparent tothose skilled in the art upon examination of the drawings and detaileddescription which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a low water cut-off system including waterprobe according to the present invention.

FIG. 2 is a block diagram of circuitry of the low water cut-off systemaccording to the present invention.

FIG. 3 is a schematic circuit diagram of the low water cut-off systemaccording to the present invention.

Corresponding reference characters indicate corresponding elements amongthe views of the drawings.

DETAILED DESCRIPTION OF PRACTICAL EMBODIMENT(S)

Referring to the drawings, one of the possible embodiments of the lowwater cut-off system according to the present invention is generallydesignated A in FIG. 1. Low water cut-off system A includes a housing 10and a threaded fitting 12 from which extends an electrical probe 14 fortesting the presence of liquid about probe 14. Fitting 12 may bethreaded to a boiler, tank or other vessel (represented by dotted lineas vessel 16 in FIG. 2) or other apparatus in which it is desired tosense the level of fluid W, such as water, or other liquid therein.

Probe 14, whether physically separate from (but interconnected with)circuitry 18, or as an integral part of housing 10, is installed intothe tank or vessel at the predetermined level (indicated by the legendLEVEL in FIG. 2) at which fluid W is normally maintained. It will beunderstood by one of ordinary skill in the art that an alarm or cut-offor other appropriate corrective, warning or indicating action is takenif the fluid level drops below the level of probe 14, resulting from theoperation of electronics 18 of the low water cut-off system A.

Referring to FIG. 2, the electronic circuitry 18 of low water cut-offsystem A injects a periodic signal into the conductive fluid W containedinside a vessel or tank (not shown) and employs probe 14 to sense thesignal as indicative of the presence of fluid W about probe 14. Probe 14then is used to effectively detect for the signal by probing for itspresence in such fluid W when so permitted by the electricalconductivity of fluid W about the probe 14. Even though fluid W iselectrically conductive it may be subject to variations in its normalelectrical conductivity as noted hereinabove. For example, theelectrical conductivity is low in water of high purity, i.e., itsresistance to electrical current is high. In operation, low watercut-off system A provides an adjustment that compensates for lessconductive, pure water applications.

As is known in the art, the boiler may be conventionally fired by aburner (not shown) to be protectively controlled by low water cut-offsystem A. Housing 10 may be of the size shown, or may be larger or muchsmaller, and its circuitry either located in housing 10 or locatedremotely from the housing, but connected in any event to probe 14. Probe14 is provided with an electrical conductive tip electrode portion 20and an electrically insulating stand-off sleeve 20 s, from which it willbe shown that tip 20 provides electrical conductivity with fluid W.

General Operation.

Low water cut-off system A generates an AC signal and transmits the ACsignal into fluid W contained within a tank or vessel. If the conductivefluid W covers the electrode in probe 14, the electronics in low watercut-off system A detect the presence of the AC signal. As long as fluidW covers the probe 14, the electronics energize a relay K1, which can beused to enable devices, such as boiler heating sources. When fluid Wdoes not cover the electrode, low water cut-off system A detects theloss of the AC signal and de-energizes relay K1, which can subsequentlydisable devices, such as boiler heating sources, thereby preventing adry fire or explosion.

Delay Operation.

In applications where short cycling of low water cut-off system A isundesirable, for example, when conditions are such that fluid W may onlytemporarily uncover probe 14 (as in the event of tank sloshing),programmable delay or delays may be enabled for low water cut-off systemA. Thus low water cut-off system A may be configured to create a delayon make, DOM, and a delay on break, DOB. The DOM setting requires thatlow water cut-off system A detect fluid W covering probe 14 continuouslyfor the specified delay setting. During the filling of a boiler withfluid W, this delay will prevent premature enabling of the heat sourcedue to turbulence in fluid W. The DOB setting requires that low watercut-off system A detect a lack of fluid W continuously for the specifieddelay setting. In steam generation applications, the turbulence due toboiling will not cause premature disabling of the heat source if a DOBsetting is enabled.

Specific Circuit Features and Operation.

Comparison of the block diagram of low water cut-off system A shown inFIG. 2 with the schematic circuit diagram of FIG. 3, illustrates boththe specific features and specific operation of circuitry 18.

In FIG. 2, a block 14′ schematically represents a functional connectionfor probe 14 and for a signal path 15 which includes a connection forproviding a signal input to fluid W from a signal generator 19associated with a power supply 100. As further shown, the intrinsicresistance of fluid W as sampled or tested by probe 14 is designated RW.Block 21 represents a conditioning circuit for probe 14. A comparecircuit is shown by block 22. A water purity adjustment for the comparecircuit 22 allows selective calibration of low water cut-off system Afor taking into account differences in electrical conductivity of fluidW, and is represented by block 23. A test circuit is shown as block 24to provide a test signal for conditioning circuit 21 for self-testing bylow water cut-off system A.

Supervision and control of low water cut-off system A is carried out bya microcontroller shown as block 25, to which is operatively connectedto an output circuit 26 for providing a control function output, e.g.,for burner cut-off, or other control signalling or functioning whichusers may wish to make of low water cut-off system A. In other words,output circuit 26 provides a suitable control function in response towhether the level-determining signal is sensed by probe 14 in order toindicate whether fluid W has a level at least as high as thepredetermined level at which probe 14 is located. The control functionmay be used for alarm or cut-off purposes if the fluid level shiftsrelative to the predetermined level, marked with the legend LEVEL inFIG. 3.

A reset circuit is shown as block 28 for resetting of low water cut-offsystem A through operation of the microcontroller 25 in accordance withdelay settings established by a delay setting circuit represented byblock 27.

Referring to FIG. 3, more specific circuit features will now bedisclosed and their operation explained in greater detail. The circuitry18 associated with conditioning circuit 21 includes circuit componentswhich make up the conditioning circuitry that receives an AC signalconducted by any fluid W, such as water, that covers probe 14. If nofluid W covers probe 14, resistors R6, R11, and R15 will have no ACvoltage applied to them. This allows R11 to pull the input of thecomparator U2B in block 22 to a low level. Conversely, if fluid W coversprobe 14, the AC voltage conducts through fluid N and appears on theresistors R6, R11, and R15, and in turn, drives the input of the comparecircuit 22. Compare circuit 22 stage squares the AC signal and sets thepulse width of its output relative to the water purity adjustment level,potentiometer RV1. If the conductivity of fluid W is low enough, suchpulsed output will have a pulse width large enough to charge capacitorC3 above the reference set by resistors R19 and R25. When this occurs,final comparator stage U2A will generate a low voltage on its output tothe microcontroller 25.

When fluid W covers probe 14, microcontroller 25 monitors the comparecircuit 22 output for a high level which would indicate then that fluidW is no longer covering probe 14. If a delay setting 27 is enabled for adelay on break (DOB), microcontroller 25 monitors the compare circuit 22output to be set continuously high for the time specified by the DOBsetting. Upon expiration of the DOB time, microcontroller 25 then turnsoff transistors Q1 and Q3, which causes relay K1 to de-energize. LED D7then illuminates when relay K1 de-energizes to visually indicate a lowfluid condition.

So long as fluid W covers probe 14, microcontroller 25 monitors thecompare circuit 22 output for a low level, which indicates that fluid Wis covering probe 14. If a delay setting is enabled for a delay on make(DOM), microprocessor U3 monitors the compare circuit 22 output todetermine if that output is continuously low for an interval of timespecified by the DOM time setting. Upon expiration of the DOM time,microcontroller 25 then turns on transistors Q1 and Q3, which causesrelay K1 to energize.

Compare circuit 22 contains a DIP switch S1 for setting delay on make(DON) and/or delay on break (DOB). The user may set either or both DOMand DOB switch to an “on” position for bringing about the desired DOM orDOB delay. Microcontroller 25 periodically reads these switches todetermine appropriate timing operation. Preferably, microcontroller 25may be a type PIC16C505.

In accordance with one aspect of the present invention, low watercut-off system A may be set for automatic reset or manual reset.Preferably, resistor R32 is a jumper installed to set the mode of resetto be implemented by low water cut-off system A.

If low water cut-off system A is installed for manual reset operation,then upon a low fluid condition, the user must manually press a resetswitch S2 (when fluid again covers probe 14) to restore system A tonormal operation. Block 28, the reset switch S2 and its associatedcomponents are only installed to provide a manual reset version of lowwater cut-off system A. Printed circuit board PCB may be used for bothmanual and automatic reset versions of low water cut-off system A.

If low water cut-off system A is installed for automatic reset,circuitry 18 is restored to normal operation immediately uponrestoration of the fluid level to an extent covering probe 14.

Block 24 contains circuits that will test the functionality of the lowwater cut-off system electronic circuits of low water cut-off system A.A pushbutton switch shunts the AC signal, if present, from theconditioning circuits in block 22. This simulates a low fluid conditionto the comparator circuit in block 22 and consequently places low watercut-off system A into a low fluid state in which relay K1 becomesde-energized and LED D7 is illuminated. The current due to the AC signalwill pass through and illuminate the test LED D8. If the AC signal isnot present, low water cut-off system A will already be in a low fluidcondition, and the test LED, i.e, D8 will not illuminate.

Block 10 contains a power supply for low water cut-off system A. Itcomprises a transformer T1 and bridge circuit (diodes D1 and D3) toconvert either 120 VAC or 24 VAC to 12 VDC, which is then down-regulatedto provide a 5 VDC output for supplying these electronic circuits.

The signal generator, shown in block 29, uses the center taps fromtransformer T1. The signal generator 29 feeds two series zener diodes D9and D10. Their outputs are capacitively coupled via C4 to the probe 14.The AC signal that capacitor C4 transmits through the probe 14 conductsinto fluid W and back to block 21 if fluid W covers the electrode ofprobe 14.

Although the embodiment description sets forth an arrangement forintroducing an AC signal and transmitting such a signal into the fluid Wcontained within a tank or vessel for detection by probe 14, it would beapparent to those skilled in the art that the probe 14 can itselfintroduce the signal and the system configured to sense whether fluid Wis a sink to the signal so introduced.

Although the signal sensed by the probe may be 60 Hz or 120 Hz or someother multiple of the main power source, the system may be designed sothat the signal may also be transmitted by radio frequency (RF) means ofa suitable frequency, and it may also be sinusoidal or of pulse-formcharacteristic. In addition, the system may be designed so that otherforms of signals, particularly those similarly of periodic character,such as pulses having an ultrasonic pulse repetition rate, or othersonic or ultrasonic or subsonic signals, may be used for probe 14sensing in accordance with the general principles herein disclosed.

It should be understood from the foregoing that, while particularembodiments of the invention have been illustrated and described,various modifications can be made thereto without departing from thespirit and scope of the present invention. Therefore, it is not intendedthat the invention be limited by the specification; instead, the scopeof the present invention is intended to be limited only by the appendedclaims.

1. A low-water cut-off system for determining if water drops below apredetermined level within a water-containing enclosure, comprising: asignal generator operative to introduce a signal into thewater-containing enclosure such that the signal is present for sensingwithin water at the predetermined level, a probe at the predeterminedlevel capable of sensing the presence of the signal within the water ifthe water is at the predetermined level, and a control responsive to theprobe sensing for providing a control function in response to whetherthe signal is so sensed by the probe, in order to indicate therebywhether water has dropped below the predetermined level wherein thesignal is transmitted through water in the enclosure according to thevalue of electrical conductivity of the water, the system including aprovision for selectively adjusting the sensitivity of the probe sensingaccording to said value.
 2. The low-water cut-off system according toclaim 1 wherein the control function is an indication that water hasdropped below the predetermined level, and including a circuitarrangement to selective set either a delay on make or delay on breaktime, or both, for indication that water has dropped below thepredetermined level.
 3. The low-water cut-off system according to claim1 wherein the signal is of periodic nature.
 4. A fluid-level detectingsystem for determining if fluid is at a predetermined level within afluid-containing space in which fluid could be at the predeterminedlevel, the fluid being transmissive of a signal, comprising: signalgenerating circuitry capable of introducing such a signal into thefluid-containing space such that the introduced signal is present forsensing within fluid at the predetermined level if, and only if, thefluid is at least as high as the predetermined level, a probe andprobe-responsive circuitry capable of sensing the presence of theintroduced signal within the fluid at the predetermined level, andcontrol circuitry capable of providing a control function in response towhether the introduced signal is so sensed, in order to indicate whetherthe fluid has a level is at least as high as the predetermined levelwherein the signal is transmitted through fluid in the fluid-containingspace according to the value of electrical conductivity of the fluid,the system including a provision for selectively adjusting thesensitivity of the probe sensing according to said value.
 5. Apparatusresponsive to presence of a fluid at a predetermined level within afluid-receiving space in which the fluid can rise to the predeterminedlevel and for providing a control function if a level of the fluidshifts from the predetermined level, comprising: a signal generatingcircuitry capable of introducing a level-determining signal into thefluid-containing space such that the periodic signal is present forsensing within fluid at the predetermined level if, and only if, thefluid level is at least as high as the predetermined level, a probe andprobe-responsive circuitry capable of sensing the presence of theperiodic signal within the fluid at the predetermined level, and controlcircuitry capable of providing a control function in response to whetherthe level-determining signal is so sensed, in order to indicate whetherthe fluid has a level is at least as high as the predetermined level,whereby the control function may be used for alarm or cut-off purposesif the fluid level shifts relative to the predetermined level ,whereinthe signal is transmitted through fluid in the fluid-receiving spaceaccording to the value of electrical conductivity of the fluid, thesystem including a provision for selectively adjusting the sensitivityof the probe sensing according to said value.
 6. The apparatus as setforth in claim 5, wherein the probe is configured for being insertedinto the fluid-receiving space at the predetermined level.
 7. Theapparatus as set forth in claim 6, wherein the probe-responsivecircuitry comprises a detector circuitry interconnected with the probefor receiving and determining receiving by the probe of the periodicsignal.
 8. The apparatus as set forth in claim 7, wherein the fluid hasa transmissivity characteristic transmissivity value representing thedegree of transmissivity of the periodic signal through the fluid, thedetector circuitry including provision for adjusting sensitivity of thedetector circuit according to said value.
 9. A method for determiningthe presence of a fluid at a predetermined level within afluid-receiving space in which the fluid can rise to the predeterminedlevel, comprising: introducing a signal into the fluid-receiving spacesuch that the signal is present for sensing within fluid at thepredetermined level, wherein said signal is transmitted through water inthe fluid-receiving space according to a value of electric conductivityof the water, providing a probe at the predetermined level for sensingsaid signal, sensing for the presence of the signal within the fluid atthe predetermined level, providing a control function in response towhether the signal is so sensed, in order to indicate whether the fluidis present at the predetermined level, and selectively adjusting thesensitivity of the probe sensing according to said value.
 10. The methodof electronically determining whether fluid is at a predetermined levelwithin a fluid-receiving space, comprising: introducing a signal intothe fluid-receiving space such that the signal is present for sensingwithin fluid at the predetermined level, wherein said signal istransmitted through water in the fluid-receiving space according to avalue of electric conductivity of the water, providing a probe at thepredetermined level for sensing said signal, sensing for the presence ofthe signal within the fluid at the predetermined level, and providing acontrol function in response to whether the signal is so sensed, inorder to indicate whether the fluid is or is not present at thepredetermined level, and selectively adjusting the sensitivity of theprobe sensing according to said value.
 11. The method accord to claim 10wherein the step of sensing for the presence of the signal is carriedout by using a signal-responsive probe inserted into the fluid-receivingspace at the predetermined level.
 12. The method accord to claim 11wherein the signal introduced into the fluid-receiving space is of aperiodic nature.
 13. The method accord to claim 12 wherein the periodicsignal is coupled to the fluid in the space from a periodic signalgenerating circuit.
 14. The method accord to claim 13 wherein the stepof sensing for the presence of the signal at the predetermined level iscarried out by using a detector circuit interconnected with the probe.15. The method accord to claim 14 wherein the fluid has a characteristichaving a value, subject to possible variation, which determinestransmissivity of the signal through the fluid, the method furthercomprising adjusting sensitivity of the detector circuit according tosaid value.
 16. The method accord to claim 15 wherein the controlfunction is an indication that fluid is lower than said predeterminedlevel.
 17. The method accord to claim 10 wherein at least the sensingand control function providing steps are carried out undermicrocontroller control.
 18. The method accord to claim 10 wherein thecontrol function is an indication that fluid is lower than thepredetermined level, and at least the sensing and control functionproviding steps are carried out by microcontroller control, and byfurther using microprocessor control to determine either a delay on makeor delay on break time, or both, for indication that fluid is lower thansaid predetermined level.